In his book On the Origin of Species, Charles Darwin boldly predicted that buried deep in the earth would be creatures with features that were intermediate between those of ancestral and modern groups. At the time no such fossils had been found, and Darwin’s critics immediately seized on that fact to refute the theory of evolution. But since Darwin’s time, many transitional fossils have been discovered, and they provide crucial insights into the origin of key structures and the creatures that possess them.

Starring University of Chicago paleontologist and award-winning author Neil Shubin, the film Great Transitions: The Origin of Tetrapods, provides a first-hand account of the painstaking search for Tiktaalik, a creature with a mix of features common to fish and four-legged animals. What made the discovery of Tiktaalik's fossil so compelling is that it, along with the series of species that existed before and after, illuminates key evolutionary steps in the transition of life from water to land.